I doubt it. Being low on coolant is far from being the ideal operating conditions. That being said I think you would need to be missing more than just a little coolant / have a lot of air in the system to cause that kind of an overheat problem.

My 3.0 cooling system failures on my old 93 started with a bad thermostat, which then lead to a head gasket failure. It was months before the headgasket completely blew. During those months it consumed a lot of coolant, and tended to spike the temp gauge under load.

When I had a fan clutch go the temp would rise in traffic and when moving slow, once the truck was at highway speeds the temp would head back to normal.

Did you buy the Montero that was on Ebay with the bent drivers front end? If so, maybe the rad has a small leak?

Ther radiator is getting close to 20 years old. From my experience with bishis since thats all I drive and own. The radiators tend to suck after about 20 years. It may be a 1992 or whatever but it's f'n 2010 and that thing is almost 20 years old and isnt going to cool like it did. You should be able to take a air hold an d pressurizxe the cylinder to 100psi or so, and if you see tonsof air bubbles while the engine is fo course dead it is a bhg or cracked head. Otherwise I'd say it's radiator. Taking your thermostat out can make the promlem worse as it can let the water flow too fast through ther adiator and you will end up not cooling it down. I tried it in a starion once and any time i drove it hard for any amount of time without the thermostat it would ioverheat. With a 180 in it was fine. A airhold from napa is less than 20 bucks if I remember, and worth every penny. It's basically jsut a hose with a spark plug setup and then a air hose adapter on the other end.s Set the pressure on you compresspor and not only can you check for air leakss you can change valve seals witht eh heads on the engine/

Pressure test the radiator/water jacket. You can rent a tester. This will show a blown head gasket or cracked head/cracked block into the water jacket.

Or you can buy the Lisle test kit for exhaust products in the coolant.

Pull the pside valve cover and see if any head bolts are loose (overheat bowed the head and stretched the head bolts to looseness, and the head gasket is gone/going).

If you do the heads, get the radiator rodded. If not rodable, recore with a 3row core for bulletproof cooling, and I betcha the fan clutch is weak, too, but it won't be the cause of high speed overheats - that's the radiator or head gaskets...

I had a condition like this in my '89 3.0L. The cause was too much Bars Leak Head Gasket Fix - which did a good temporary fix on a head gasket problem but clogged the bottom of the radiator after 3 applications over 12 months. Too much gunk. After a rodding and rebuilding the radiator - no more temperature problem. You may have purchased a truck with a similar treatment already done. I recommend you have your radiator checked, flushed and rodded. (Small Bucks) If you don't, you run the risk of overheating and blowing your head gasket. (Big Bucks)

i had the same problem with the heat. it was the stat for me. and i modded the ac fan to turn on by switch since it never turned on anyway. if it was a head gasket check the oil for coolent in it if there is then that is ur problem. are u seeing any smoke coming from the engine then it might be underneath the injector rail and it can be a oring leak on a pipe going to the rear of the block. is ur carpet wet with coolent on the drivers side. if so then it is ur heater core....it can be a number of things. hope this helps